The expression “3D glasses” as used herein means glasses intended to be worn by a viewer to create a three-dimensional viewing experience when viewing displayed images. 3D glasses are used for various 3D display and cinema applications in order to project different images to a viewer's left and right eyes. There are various types of 3D glasses available including those that separate the left and right images using principles based on linear or circular polarization, active shuttering and colour separation. For the latter mode of image separation, different wavelength bands of light are directed to a viewer's left and right eye via a projector or a display and the 3D colour separation glasses transmit the desired wavelength bands (for the image of one eye) and block the undesired wavelength bands (for the image of the other eye).
The earliest versions of 3D colour separation glasses relied on just a single band; however, more recent versions employ three or more bands per eye in order to achieve a good colour balance for the images for each eye. Each eye therefore sees a full colour image, but using different wavelength bands for the primary colours. The simplest 3D colour separation glasses employed absorption in the lenses—typically a colour plastic film was used. With three or more colour bands and the need for precise wavelength alignment of the wavelength bands, optical interference filters are usually employed to achieve the colour separation by having substantial transmitting and reflecting bands for the passband and blocking bands, respectively. For example, a 3D coated lens for a given eye may be required to transmit wavelength bands [λS1-λF1], [λS2-λF2] and [λS3-λF3] and block wavelength bands [λS4-λF4], [λS5-λF5] and [λS6-λF6] by reflecting them, where λn and λn are the start and final wavelength boundaries of the nth band. The 3D coated lens for the other eye is then required to transmit the wavelength bands [λS4-λF4], [λS5-λF5] and [λS6-λF6] and block the wavelength bands [λS1-λF1], [λS2-λF2] and [λS3-λF3] by reflecting them. In this manner, a viewer's eyes see only the separate desired images thus allowing the viewer to see a 3D image. Inherent in optical interference filters is that fact that the spectral bands shift towards lower wavelengths as the angle of incidence of light on the lenses increases. This can limit the field of view for optical interference filters as light from a high field of view has a corresponding high angle of incidence such that desired wavelength bands of light can be now be reflected instead of transmitted while the undesired bands of light can now be transmitted instead of reflected.
The optical interference based filters currently used in 3D colour separation glasses consist of a stack of substantially non-absorbing thin film materials in order to maximize the transmittance of light in the bands that need to be transmitted to a given eye. The thin film materials are arranged with adjacent layers have alternating high and low refractive indices. The undesired bands of light are blocked primarily by reflectance and not by absorption. Such high reflectance 3D filters give rise to unwanted reflections of ambient light, which can cause a visual distraction. In addition, the 3D optical coatings are usually deposited on the concave side of curved transparent lenses with a typical radius of curvature from 60 to 140 mm. Ambient light can reflect off the concave surface and into the viewer's eye. As a consequence, a viewer wearing 3D colour separation glasses can see a substantial reflectance of ambient light off the back (concave) surface of the 3D coated lens and also see an image of their face and eyes reflected back to them. This back reflectance effect can be distracting and is worse if the viewing room is not completely darkened. In addition, light from the projectors or displays can be reflected off nearby ambient objects and be reflected back directly to the viewer's eye or indirectly by reflecting off the viewer's face back to the coated lens and then back to the viewer's eye.
The amount of back reflectance of the 3D optical filter coated concave side of a lens can be slightly reduced by applying an anti-reflectance coating to the convex surface of the lens. The amount of light that is reflected back to a viewer's eye is then reduced by 3 to 4%. However, this is a small percentage relative to the total light reflected back towards the viewer's eye by the 3D optical filter and in practice does not significantly reduce the back reflectance distraction.
In another approach, the back reflectance distraction can be minimized by designing the glasses frames such that the ambient light, whether from the projector or another light source, is blocked from reaching the back of the coated lens. However, even with a good frame design, sufficient ambient light is usually able to reach the coated lens to create a visual distraction.
A further distraction can arise as a result of some of the desired light transmitted through the coated 3D lens being reflected off the viewer's face. As mentioned previously, the spectral bands transmitted by the coated lens are shifted towards lower wavelengths as the angle of incidence on the coating increased. The transmitted desired light that has been reflected off a viewer's face can be then be incident upon the back surface of the 3D coated lens at such an angle that this light is now reflected back towards viewer's eye creating a visual distraction.
With cinema applications using a Xenon-based projection, the visual distraction of the back reflectance was such deemed acceptable with suitable glasses frames and a darkened viewing room. However, with the advent of laser projection (“6P”) based cinema, the amount of light being projected is more than doubled. With 6P cinema, three laser bands (Short-Blue, Short-Green and Short-Red) are dedicated to one eye and three other laser bands (Long-Blue, Long-Green and Long-Red) are dedicated to the other eye. With the increased light being projected there is more indirectly reflected laser light getting to the back side of the coatings as well as more desired light transmitted through the coated lenses. As explained above, this coated lens back reflectance effect can result in a significantly worse distraction (i.e., viewer's face around the eyes being visible) for 6P projectors compared to Xenon projectors.